The present invention, generally, relates to igniter system for a jet engine system and, in particular, relates to silicon nitride-encased hot surface igniter system for jet engine system.
The four basic parts of a typical jet engine are the compression section, turbine, combustion section and propelling nozzles. In general, a jet engine can be comprised of the following working cycles: The compression cycle where work is done to increase pressure and decrease the volume of the air resulting in a corresponding rise in temperature. The combustion cycle when fuel is added to the air and ignited to increase the temperature causing a corresponding increase in volume while the pressure remains almost constant. The expansion cycle where there is a decrease in temperature and pressure with a corresponding increase in volume. This volume is exhausted to provide thrust. Newton's third law of motion requires that the force that causes the high-speed motion of the jet of gas have a reaction force that is equal in magnitude and oppositely directed to push on the jet propulsion engine. There may be multiple stages of compressors (compressing the air flow) or turbines (extracting energy from the airflow).
There are several types of common jet engines. For example, a typical thermal jet propulsion engine operates with a continuous blast of thrust-producing, expanded heated gas. In contrast, in an intermittent duct jet propulsion system, thrust is generated by a series of pulses, or intermittent explosions. A ramjet, or continuous duct, engine relies on its own forward motion to compress the air that enters it. Although highly efficient, it is designed to operate only after high speed has been attained through the use of some other power source, typically a rocket. A scramjet, or supersonic-combustion ramjet, engine is designed to operate at hypersonic speed (i.e., above Mach 5), using hydrogen for fuel. In theory, a scramjet-propelled craft could achieve orbital speed, with an efficiency three times that of liquid- or solid-fuel rockets. In addition, without the need to carry oxygen, an air-breathing, scramjet-powered vehicle can carry a greater payload than a rocket-powered one.
Additionally, there are various thrust-augmentation methods that can be used to increase the effective driving force of jet propulsion engines: the afterburner, water-injection, and air bleed-off methods. An afterburner uses the exhaust gases from the engine for additional combustion, with resulting higher compression. However, the afterburner consumes large amounts of fuel. Injection of water into the air-compressor inlet also increases the thrust, but can be used only at take-off because of the high water consumption. Air bleed-off, sometimes called the fan augmentation method, also makes more efficient use of air otherwise wasted.
Other common forms of engines include high power-to-weight ratio, turboshaft style of jet engines, specifically gas turbine engines, that are used in many other applications beyond directly thrusting aircraft through the sky. As examples, the current generation Abrams M1-A2 main battle tank uses a 1500 HP gas turbine engine to provide power to a hydrokenetic transmission. Additionally, many U.S. Navy ships use the combined output of twin gas turbine engines, derived from a successful family of aircraft engine designs, to provide power to controllable pitch propellers through a main reduction gear, shaft and clutch. Also, turboprop aircraft use gas turbine engines as their powerplants as do many types of rotorwing aircraft (helicopter). For these types of aircraft, the turbine section provides torque to a shaft which spins one or more propellers or the main and tail rotor blades to propel the aircraft through the air. In many aircraft designs, the hot exhaust gases may be directed to provide a degree of supplementary thrust. In addition, large transport category aircraft such as Boeing 747s have at least one relatively small gas turbine engine for use as an auxiliary power unit (APU) to generate electric power and compressed air for ground-based activities prior to main engine start and during in-flight emergencies. Finally, stationary turboshaft power plants are used worldwide as mission-critical primary and/or emergency back-up generators or as prime movers for powering pumping stations for water or other fluids.
Typically, ignition in conventional jet engine designs can be accomplished by incorporating a high tension spark gap into a combustor element or, in the case of turbine engine augmenters (i.e., afterburners), the spark igniter is inserted into a fuel enriched flow stream. These systems have successfully been employed since the early development of the turbine engine. Spark igniters are provided in various locations and geometries, using different voltages and spark rates. Igniter systems employing high temperature elements in jet engines are also known. These metal encased units (commonly referred to as “Glow Plugs”) typically require very large currents in order to maintain temperature. Additionally, the metal surfaces can also suffer embrittlement. Hence, these designs have not been widely employed in initiation of combustion for turbine engines.
Silicon nitride (Si3N4), an advanced ceramic, was developed in response to a need for a high strength and high temperature material. The initial application of silicon nitride was to replace metals with ceramics in advanced turbine and reciprocating engines to give higher operating temperatures and efficiencies. Even though a totally ceramic engine has never been achieved, silicon nitride has been used in a number of industrial applications, such as engine components, bearings and cutting tools. Silicon nitride has better high temperature capabilities than most metals combining retention of high strength and creep resistance with oxidation resistance. In addition, its low thermal expansion coefficient gives good thermal shock resistance compared with most ceramic materials. These characteristics make it an ideal ceramic for igniter technology.
Hot surface igniters that comprise a Silicon Nitride-encased resistive element have been employed in consumer and industrial applications to initiate combustion in stoves and furnaces for many years. The first application was as early as 1965 in the electronics industry. When voltage is applied to the hot surface igniter, it heats up which in turn causes combustion. However, standard existing hot surface igniters have not been able to achieve ignition in jet engines due to the convective cooling of the igniter in the airstreams typically associated with a jet engines.
Commercially available single cylinder and planar-shaped hot surface igniters elements for heating, ventilation, air conditioning (HVAC) systems are used primarily to light-off the pilot light which subsequently provides the source of ignition for the larger main burner(s) of the heating system. As such they operate in a nominally static (zero flow) environment using ambient air. At any particular voltage level, the current specification is set by hard wiring to a power source or via some sort of active power management system designed to provide power at levels (a) high enough to operate at a temperature above the auto ignition temperature of the fuel and (b) as low as possible to prolong the life of the wiring and resistive element(s) of, typically, a single element. Also, operating conditions do not significantly change in terrestrial installations as they do in aircraft.
The flow environment within a jet engine's combustion section down stream of the compressor section is not static but instead extremely fast and highly turbulent. The forced convection cooling effects of the air flow directly impinging upon a heated hot surface igniter's element remove so much heat that an unprotected element's encasement temperature immediately falls below the auto ignition temperature (i.e., no ignition). To compensate for this forced convective cooling, the internal resistance heating element(s) will need to be “overdriven” with an increased power such that—in a no flow or low flow fluid environment—would be of an amount of power to cause (immediate) failure of the internal heating element(s) and/or connective wiring. Jet engines used to generate thrust or torque in mission-critical applications, will, for the most part, have a plurality of igniter assemblies and a plurality of igniter controllers to achieve operational redundancy.
Therefore, there is a need for a silicon nitride-encased, hot surface igniter system for a jet engine system that can withstand the turbulent airstreams conditions encountered within an operating jet engine.
There is a need for an active power management system within each igniter control system to provide optimized levels of conditioned power depending upon RPM. Operational command conditions such as an auto-relight activation command from a Full Authority Digital Engine Controller in foul weather conditions may be employed to (a) ensure ignition and (b) prolong component life of the plurality of igniter assemblies within the at least one combustion section of the engine.